Remove the watchdog's fd tracker.

The existence of both fdtrack and the older OpenFdMonitor is confusing
to partners, and not obviously useful because the fdtrack limit is so
much lower than the OpenFdMonitor limit (now that the default
RLIMIT_NOFILE is 32Ki).

Bug: https://issuetracker.google.com/161199049
Test: builds
Change-Id: I86436f287d0e25d1dfae95b915beb78c88dccce2
Merged-In: I86436f287d0e25d1dfae95b915beb78c88dccce2